Speed control of induction motors



June 8, 1954 E. c. RHYNE, JR., Er A1. 2,680,829

SPEED CONTROL OF INDUCTION MOTORS Filed Oct. 6, 1951 2 Sheets-Sheet l l'-011 u u m h nog oo u? h a. h23 l u u m @"1, I m

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l al 6| III I INVENTORS Eorl C. Rhyne Jr.,Herber1 R. Behr 8| Milton L.Priban.

ATTORNEY June 8, 1954 Filed OCT.. 6, 1951 Per Centl Synchronous SpeedPer Cent Synchronous Speed E. C. RHYNE, JR., El AL SPEED CONTROL OF'INDUCTION MOTORS 2 Sheets-Sheet 2 Fig.4.

Per Cent Synchronous Speed INVENTORS Jr. Herbert R. Behr 8 Milton L.Prbon.

ATTORN EY Een C, Rhyne Torque Patented June 8, 1954 UNITED STATES PATENTOFFICE SPEED CONTROL OF INDUCTION MOTORS of Illinois Application October6, 1951, Serial No. 250,112

13 Claims. (Cl. S18-201) Our invention relates to an electric system ofcontrol for controlling electric motors, and more particularly ourinvention relates to a system applicable to control an electric motor ofthe induction type and the load coupled thereto.

Further, to state to what our invention relates very brieiiy but yetfairly comprehensively, our invention includes a generic control systemutilizing saturable core reactors between the supply of alternatingcurrent and an electric load but wherein the load comprises a particulartype of motor, an induction motor and its mechanical load, to obtainproper motor acceleration, normal selected constant speed running,selected slov.7 speed running, inching operation of the motor from acondition of rest, and stopping of the motor by means of direct currentbraking.

The generic control above mentioned is, per se, not part of thisinvention, but such generic conu trols are disclosed and claimed in thecopending applications of Earl C. Rhyne, Jr., Serial No. 189,220, ledOctober 9, 1950, and entitled Control Systems for Alternating CurrentMotors, and Serial No. 250,114, led on the same date as this applicationand entitled Control Systems for Alternating Current Motors.

One broad object of our invention is the combining of a genericsaturable reactor control with the induction motor to be controlled toprovide a complete and commercial product.

Control systems utilizing saturable reactance devices between thealternating current supply and the primary of the motor are well known,but if the pre-saturation of such devices is normally constant, thecontrol performance is not suitable because the speed-torquecharacteristic of the motor is so modied that a relatively high speedpull-out point exists.

We utilize a saturable core reactor type of control having specialfeedback features and thus avoid a high speed pull-out. One specialfeature of our control, by way of illustration, is the provision ofautomatic recalibration of the control system with changes of impedanceof the motor secondary circuit.

It is another broad object of our invention to provide any selectedconstant speed operation of an induction motor, and thus its load, bymerely selecting the desired speed setting on a control circuit of thesaturable core reactors utilized, with other suitably energized controlcircuits providing the necessary stabilizing components to regulate forconstant speed.

Another object of our invention is to improve the sensitivity andefficiency of the regulating performance of such control systems so thatthe controlled series impedances vary more sensitively and over agreater range in response to a change in motor speed, to provide acontrol that is applicable to a greater variety of loads and inparticular a printing press load, and to provide such improved accuracyand eiciency with fewer and smaller components as compared with somewhatsimilar known systems of control.

The objects recited are merely illustrative. Still other objects andadvantages will become more apparent from a study of the following moredetailed description of our invention and from a study of theaccompanying drawings, in which:

Figure 1 is a diagrammatic showing of our system of control as appliedand combined with a single motor driving a printing press;

Figs. 2, 3 and 4 are coordinate speed-torque curves explanatory of theoperation of our control system.

In Figure l, the motor to be controlled is designated by M, the mainalternating current energizing terminals are shown as leads L1, L2, andllc and 2|, 3|, and 4| designate the saturable core reactors disposedbetween the leads L1, L2, and L@ and the primary i9 of the motor M.

The motor to be controlled is a wound rotor induction motor having theresistor sections 86, 8l, S8, ||6, ll'i and H8 in the external portionof the secondary winding, or rotor portion, of the motor. The externalsecondary circuit also includes the capacitors |26, |21, and |28. Thepurpose of the capacitors will be explained hereinbelow.

The saturable reactors 2|, 3| and 4|, include the main alternatingcurrent windings 2D, 30 and 40, respectively, series connected with theprimary winding i9, and include the reactor-saturating control windings22, 23 and 24 and 32, 33 and 34, and 42, 43 and 44, respectively. Thewindings 22, 32 and 42 are the pattern windings and to provide effectiveselected constant speed for the motor include in their circuit the speedcontrol rheostat Rh.

The windings 23, 33 and 43 provide a reactor saturating component thatis a function of both the armature, or primary, terminal voltage and thesupply voltage, and windings 24, 34 and 44 provide a reactor-saturatingcomponent that is a function of the armature, or primary current.

The transformer |03 and the rectier IGI coupled thereto provide directcurrent braking of the motor M at stopping; the switch SP provides slowspeed or trip slow operation of the motor from an operating or runningspeed con- 3 dition of the motor, and inching switch IS provides veryslow inching speed for the motor from a condition of rest oi the motor.

To understand the purpose and advantages of the capacitors I26, I2? andIES a brief cxplanation of the curves shown in Figs. 2, 3 and i may behelpful.

As mentioned, when an attempt is made at controlling the speed of aninduction motor by series impedances in the primary motor circuit With axed impedance adjustment, the obtainable stable speed control range israther limited. This will be understood from the family oi speedtcrquecurves shown in the diagram of Fig. 2. This diagram corresponds to testresults taken with a test system similar to the one shown in Fig. 1, butusing only pattern windings such as 22, 32 and 42 with adjusting meansfor such windings. Capacitors such as |26, IZ'II and t28 were not used.rThe impedances of the reactor main windings of the test system werecontrolled only by a selected fixed adjustment of a constantdirectcurrent bias applied by windings such as 22, 32

and 42. The curves kshown in 2 are denoted by values of 0.75, 1.0, 1.25,etc., which quantities represent the fixed amounts of direct-currentbias applied to the windings 22, E2 and d2. It will be observed thatexcept for a limited range of high bias currents, the speed-torquecurves have a negative slope and hence indicate motor characteristicswhich are unsuitable, for speed control.

In Fig. 3 the speed-torque curves shown in full lines depict motorcharacteristics obtained by applying selected constant values of voltageto f the motor terminals. It will be seen that these curves havepositive slopes indicating that speed control of a motor having suchcharacteristics is feasible. For instance, if the motor tnus controlledis to operate at a constant load torque denoted by line F, the speed ofthe motor is definitely determined by the intersection (point X) of theload torque line F with the one selected speed-torque characteristic.Consequently, in a system according to Fig. 1, which regulates theseries impedance to maintain a selected motor terminal voltage constant,the just-mentioned conditions permit the selection of deiinite speedvalues simply 4by adjusting the control rheostat Rh in accordance withone of the available speedtorque characteristics.

As far as explained, the performance of the system is independent of thecapacitive shunts in the secondary circuit of the motor, and indeed issufficient ior many'appiications. provision of the capacitive shuntcircuits permits further improvements, as will be explained by means ofFig. 4. The speed-torque curve Q in Fig. 4 typiiies the performance ofan induction motor with relatively low resistance in the rotor circuit.As mentioned, due to the negative slope of this characteristic, a givenvoltage does not give a definite speed for a given torque below thepull-out speed (about 40% speed in the illustrated example).Consequently, the speed cannot be sufliciently controlled by primaryvoltage regulation except over a narrow range of speeds close tosynchronism.

Thecharacteristic, as regards speed stability, can be improved byincreasing the resistance lin the rotor circuit, thus obtaining forinstance the characteristic represented by curve RC. Such acharacteristic can be controlled by primary voltage regulationtheoretically down to zero speed, but for a given load torque, asrepresented However, the -i capacitor I 4 by the line F of Fig. 3, themaximum speed obtainable becomes rather limited (60% to 70% ofsynchronous speed in the illustrated example). It will, therefore, beunderstood that in order to obtain a wide range of speed control, acombination utilizing both a primary voltage control and a coordinatedchange in secondary impedance Vmight be desirable. Indeed it will beobvious that in systems according to the invention, a change insecondary resistance can readily be coordi- -nated with a change inadjustment of the control rheostat, for instance, by means of a masterv"controller jointly controlling the primary and secondary motorcircuits. However, when the secondary motor circuit is given acapacitive component, as illustrated in Fig. l, the available range ofspeed control can be greatly increased without necessitating a change ofthe secondary circuit connections. The above-mentioned characteristic Qobtained with a comparatively low resistance in the secondary rotorcircuit is modified by the capacitive shunt circuits so as to assume theshape of the characteristic This modified characteristic extends theavailable range of speed control down to near Zero speed. Thebroken-line extensions of the curves shown in Fig. 4 exemplify themodied speed-torque characteristics obtainable by means of parallelcapacitors in the secondary motor circuit. The provision of suchcapacitors has also the advantages oi a better power factor, and ahigherefliciency at low speeds.

In order to secure the above-explained increase in range of speedcontrol, the shunt capacitors in the secondary motor circuit must begiven a sufficiently high capacitance rating. The most favorable ratingdepends upon the impedance and circuit conditions of the particularapplication.

A still better understanding of our invention -may be had from a studyof its operation. Let

the assumption be that lines L1, Lz, and `L3 are Acoupled toa suitableconstant Voltage constant frequency supply of alternating current, thatthe kwound rotor induction motor M is coupled to its load, for example,a printing press, and that the attendant wishes to start and operate themotor `subject to the control provided. The start switch isoperatedwhereupon a circuit is established from lead L1 through conductor I,stop switch 42, start switch 3, actuating coil 4 of the control itor i),adjustable resistor 'I, rectier 8, actuating vcoil 9 of the ltime delayrelay TD to conductor v5 and line L2.

Acapacitor ID of a selected capacity is connected in parallel to theactuating coil 9 of time delay'relay TD and a leakage resistor Iil of arelatively high resistance value is connected in parallel to both thecoil 9 and the The-capacity of the capacitor I0 is so selected that thetime delay relay TD'has both a time delay oi a selected Value on pick-upand a time delay on drop-out.

The energization of coil 4 causes the operation of the control relay Rto close the contacts I2, i 5 and 55, and the opening of contacts 92,93, 94 and 95. The closing of contacts I2 establishes a holding circuitfor coil 4, and the closing of contacts .I5 establishes a circuit acrossconductors VI and 5 including stop switch l2, conductor I3, actuatingcoil I4 of the main contactor vMC, and contacts VI5 of relay R.

contacts I6, |1 and I8 connecting the motor primary winding I9 to thesupply of alternating current through the main, or alternating currentwindings 2li, 30 and 40 of the saturable core reactors 2|, 3| and 4|.The motor thus operates its load.

The voltage applied to the motor at the instant of the closing of thecontacts i6, |1 and I8 is rather low because the reactors, beingunsaturated, have a relatively high 'reactance The starting torque is,of course, also innuenced by the value of the eiective impedance of themotor secondary winding. To obtain the maximum starting torque thecontacts 8|] and 8| are in open position initially so that all of theresistor sections 86, 81, 88, H8, ||1 and ||8 are in the secondarycircuit.

The instant after both the control relay R and the main contactor MChave operated, a circuit is established from the energized conductor 5D,through the reactor saturating windings 22, 32 and 42, conductor 5|,contacts 52 on the high speed contactor HS, a portion of the rheostatRh, contacts 53, of the slow speed switch SP,

contacts 54 of the inch switch IS, contacts 55 of the control relay R,conductor 5B, and rectifier 51 to the energized conductor 58. Areactorsaturating current Ip is thus caused to flow in the windings 22,32 and 42. The degree of saturation of the reactors 2 I, 3| and 4| isdetermined by the position of the arm on the rheostat Rh. When thecontacts MC first close, there is no D.C. saturation current in thereactors and (except for the transient) only a low voltage is applied tothe motor terminals since the voltage drop across the series reactors islarge. Thereafter the current Ip builds up as determined by rheostat Rhto decrease the circuit reactance and increase the voltage applied tothe motor terminals.

With the energization of conductors 62 and 58 another circuit isestablished including conductor 63, winding 54 of a control, or signalmixing reactor 66, conductor 65, conductor 56, and rectiiier 51. acurrent that is a function of the supply voltage. The signal mixingreactor 66 has a second direct current winding 61 that is energized,through rectifier 68, from the motor primary terminals 60 and 6|, asshown. The direct currents in the windings B4 and 51 are so applied tothese windings that their magnetic uXes are in opposition in the core,as shown by the arrows adjacent these windings 64 and 61.

The alternating current energization of the e signal mixing reactor 86is eiected by the transformer T. This transformer has its primarywinding 89 connected to the energized conductors 58 and 63 and thesecondary winding 10 is connected in a closed circuit includingthe A.C.terminals of rectier 1| and the alternating current winding 12 of thereactor 56. From the circuits just traced, it is apparent that thedirect current output of the rectifier 1| is a function of both, thesupply Voltage and the primary terminal voltage. The output of therectifier 1| is connected, as shown, in a loop circuit including thesaturable core reactor windings 23, 33 and 43'. The windings 23, 33 and43 are poled to aid the windings 22, 32 and 42.

To provide the reactors 2|, 3| and 4| with a saturating component thatis a function of the armature current, we connect the currenttransformer CT across the rectifier 13 and connect the output circuit ofthe rectiner 13 in a loop circuit The winding B4 is thus supplied with6i with the reactor control windings 24, 34 and 44. The windings 24, 34and 44 are poled to aid the main or pattern windings 22, 32 and 42. Thepoling is indicated by the arrows adjacent the windings.

When the motor M is to be operated at high speed, the arm of rheostat Rhis moved clockwise to a greater extent than shown so that the arm 16bridges the segments 15 and 11 to thus establish a circuit from theenergized conductor |3, through contacts 14 of the slow speed switch SP,segment 15, arm 16, segment 11, actuating coil 18 of the high speedcontactor HS and conductor 19 to the energized conductor 5.

Operation of the high speed contactor HS causes the closing of contacts88 and 8| and the opening of contacts 52. The opening of contacts 52causes the insertion of the adjustable resistor 83 in the circuit of thepattern windings 22, 32 and 42. This adjustable resistor 83 recalibratesthe rheostat Rh for the changed secondary resistance effected by theclosing of contacts 8i! and 8|, the aim usually being by suchrecalibration to avoid abrupt speed changes as the high speed contactoroperates and to obtain the higher and higher speeds by further clockwisemovement of the rheostat Rh.

It will be more apparent how abrupt speed changes are avoided byreference to the curves shown in Fig. 3. If the motor were operating atpoint :t on the speed torque curve A just prior to operation of the highspeed contactor HS, and right after operation of the high speedcontactor HS, is shifted to the curve B tending to drive f load F atspeed Z. Then, if abrupt speed changes are to be avoided, the insertionof the resistor 83 in the circuit of the pattern windings 22, 32 and 42of the reactors 2|, 3i and 4| effects a transfer from point y on curve Bback to point :r on curve A (which curve A now represents a diiferentvoltage). Of course, the speed at the change-over point can be set bymerely adjusting the resistance value of resistor 83 to vary the seriesreactance in the primary circuit of 'the motor, and preferably should besuch that the speed immediately after the contactor 52 operates isexactly the same as the speed immediately before the contactor operates.

If slow motor speed is to be effected with'the motor in operation, thenthe slow speed switch SP is operated to open the contacts 53 toinsertthe slow speed adjustable resistor 84 in the circuit of the patternwindings, and contacts 85 are closed to completely shunt out therhecstat Rh. The contacts 14 are opened to effect insertion of theresistor sections 86, 81, 88, H6, ||1 and ||8 in the motor secondarycircuit, to in part effect slow speed operation by secondary resistancecontrol. The resistance Value of the adjustable resistor 814 is selectedto eifect the appropriate slow speed operation.

If, With the motor at rest, the motor M is to be operated at inchingspeed, then the inching switch IS is operated to open the contacts 54 toinsert the inching resistor 89 in the circuit of the pattern windings.The inching switch also operates contacts 90. The closure of thesecontacts 90 effects energization of the main contactor MC independentlyof the start switch push button 3. Inching, except for very rareapplications, is always started from a stopped condition of the motor.The particular control herein disclosed applies particularly to a singlemotor printing press. In such application of our control it is veryimportant that it shall notbe possible to effect .normalmotor runningoperation: by meansv ofV- the-,inching switch. Since start switch` 3 isopen and the motor is startedfrom rest, neither the control-relay R northe time'delay. relay TD can be energized, but the main contactor MC isenergized as long, and only as long, as contacts Sv are closed.

If themotor is tobe stopped from normal operation,- the stop switchZ isactuated which eiectsv the opening of the energizingy circuits for coil4 of theicontrol relay Recoil I4 of themain contacton MC, andthe coil9'of the time delay relay TD; Since neither the relay R nor the maincontactor MC' have any appreciable time constant, the contacts i5, Vland IB, on the maincontactor and contacts i5, l2 and 55 on the relayRare openalmost instantly, and the contacts 9 I-on-the main contacter,and contacts 92, e3, 94 and 95 on the control relay R close'almostinstantly, but the contacts. 96 and 91V on the time delay relay TDremainclosedior a selected time-interval.- The time constant on. dropoutof the relay TD' is matched to t the braking period required tol stopthe motor and itsload.

The closing of contacts 9de-establishes acircuit from the energizedconductor l through theactuating coil 98 of the braking relay BR,includingv contacts. lllv of the time delay relay TD, to the conductorThe braking relay BR now closes its contacts 99 and |00. A brakingcircuit is thus established from the positive terminal of; the rectifierIl through thecontacts 92, S. al, conductor 5G, winding 2B, terminal 5t,the motor primary winding I9, terminait i, winding 55B, conductor 58,contactsy |00 and 93 to the negative terminallof the rectiier li.The-rectifier HH is energized through contacts Sii, from the secondarywinding |02 of the transformer H33. The transformer primary ifi isconnected directly across the leads L1 and L2 as shown.

Although butone embodimentof this invention has been hereinillustratedand described, it will be appreciated by those skilled in theart that various modifications may be made in system details without,departing from the spirit and scope of; this invention. Accordingly, itis intended. that the foregoing disclosure andthe illustrations in thedrawings shall beg consideredy only as illustrative and not interpretedin.. a limiting sense.

We claim as our invention:

1. In a system ofcontrol for an induction motor, inlcombination, aninduction` motor having a primary winding and a secondary winding, said,secondary winding including speed control resistors in the secondary`circuit, a saturable reactanee means having a main alternatingv currentwinding series` connected with the primary winding and having aplurality of direct-current reactor-saturating control windings, adirectcurrent source of adjustable constant, voltage connected toA afirst of saidA reactor-saturating windings,v a speed-controlling andispeed-setting rheostat-1 connected to adjust thevoltage supplied to;said first reactor-saturating winding, an ad,- justable resistor in thecircuit of said 'first reactor-saturating winding and a slowK speednorn'iallyl closed switch shunting the adjustable resistor, wherebyoperation of the-slow speed'switch to open` position inserts theadjustable resistor in the circuit ofthe firstreactor-saturating;winding, normally open contactors for shunting thespeed control resistor in the secondary circuit, electromagnetic meansfor eiectingI the closing of. the cont/actorsv for shunting, theresistor sec-v tions in; the secondary circuit, a second of, saidreactor-saturating windings for providing a con.- trol component tothesaturable reactors proportional tothe-loadcurrent of the motor,saidspeed control resistors inI thesecondary circuit being operable tochange the slope of the speed-torque characteristic of the motor.

2; n a system of control for an induction motor, in combination, aninduction motor hav,- ing a primary winding anda secondary winding,

saidsecondary winding including spec/d controlA resistcrsinlthesecondary circuit, asaturablere.-

actance means having a main alternating current` winding seriesconnected with the primary winding and having a plurality ofdirect-current reactor-saturatingcontrol windings, a directcurrentsource of adjustable constant voltage connected to a first of saidreactor-saturating windings, a speed-controlling and speed-settingrheostat connected to adjust the Voltage supplied togsaid rstreactor-saturating winding, an adjustable resistor in the circuit ofreactor-saturating winding and an inching switch normally closed toshunt said adjustable resistor,

whereby operation of the inching switch to open position inserts saidadjustable resistor in the circuit of said first reactor-saturatingwinding,

normally open contacts for shuntingA the speedv control resistor inthe-secondary circuit, electromagnetic means for effecting the closingof the contactors for shunting the resistor sectionsV in the secondarycircuit, a second of said reactorl saturating windings for providing acontrol component to the saturable-reactors proportional to theloadecurrent of the-motor, said speed control resistorsin the secondarycircuit being operable.

to change the slope ofl the speed-torque characteristic of the motor.

3. In a system of control for an induction motor, in combination, aninduction motor havinga primary windingY and a secondary winding,

said secondary winding including speed control` able resistor, saidsecond adjustable resistor in the circuit of the firstreactor-saturating winding, a normallyclosed contacter shunting at leasta part. of one ofv said resistors, normally open contactors for shuntingsaid speed control resistor inthe secondary. circuit, electromagneticmeans for effecting the opening of the first named contactor, torecalibrate the effect of the rst reactor-saturating winding, saidelectromagnetic means effecting closing of the second named contactorsfor shunting the resistor' sections in the secondary circuit, a secondreactor-saturating winding for providing a control component to thesaturable reactors proportional to the load current of the motor, saidspeed control resistors in the secondary circuit being operable tochange the slope of the speed-torque characteristic of the motor.

.I na system of control for an induction motor, in combination, aninduction motor havsaid first ing a primary winding and a secondarywinding, said secondary Winding including speed control resistors in thesecondary circuit, a saturable reactance means having a main alternatingcurrent winding series connected with the primary winding and having aplurality ci direct-current reactor-saturating control windings, adirectcurrent source of adjustable constant voltage connected to a firstreactor-saturating winding, a speed-controlling and speed-settingrheostat connected to adjust the voltage supplied to said iirstreactor-saturating winding, normally open contactors for shunting thespeed control resistor in the secondary circuit, electromagnetic meansfor effecting the closing of said contactor-s for shunting the resistorsections in the secondary circuit, a second of said reactor-saturatingwindings for providing a control component to the saturable reactorsproportional to the load current of the motor, a third of saidreactor-saturating windings for producing a control component that is afunction of the primary terminal Voltage, said speed control resistorsin the secondary circuit being operable to change the slope of thespeed-torque characteristic of the motor.

5. In a system of control for an induction motor, in combination, aninduction motor having a primary winding and a secondary winding, saidsecondary winding including speed control resistors in the secondarycircuit, a saturable reactance means having a main alternating currentwinding series connected with the primary winding and having a pluralityof direct-current reactor-saturating control windings, a directcurrentsource of adjustable constant voltage connected to a iirstreactor-saturating winding, a speed-controlling and speed-settingrheostat connected to adjust the voltage supplied to said firstreactor-saturating winding, normally open contactors for shunting saidspeedcontrol resistors in the secondary circuit, electromagnetic meansfor effecting the closing of the contactors for shunting the resistorsections in the secondary circuit, a second of said reactor-saturatingwindings for providing a control component to the saturable reactorsproportional to the load current of the motor, a -third of said reactorsaturating windings for producing a control component that is a functionof the primary terminal Voltage and a function or the voltage of thealterhating-current supply to said main winding, said speed controlresistors in the secondary circuit being operabie to change the slope ofthe speedtorque characteristic `of the motor.

6. In a system of control for an induction motor, in combination, aninduction motor having a primary winding and a secondary winding, saidsecondary winding including speed control resistors in the secondarycircuit and including capacitors in the secondary circuit, a saturablereactance means having a main alternating current winding seriesconnected with the primary winding and having a plurality ofdirect-current reactor-saturating control windings, a directcurrentsource of adjustable constant voltage connected to a iirst oi saidreactor-saturating windings, a speed-controlling and speed-settingrheostat connected to adjust the voltage supplied to said firstreactor-saturating winding, a rst adjustable resistor in the circuit ofsaid first reactor-saturating winding and a slow speed normally closedswitch shunting at least a portion of said adjustable resistor, wherebyoperation of the slow speed switch to open position inserts the shuntedportion of said adjustable iii) resistor in the circuit of said firstreactor-saturating winding, a second adjustable resistor in the circuitof said iirst reactor-saturating winding and an inching switch normallyclosed to shunt at least a portion of said second adjustable resistor,whereby operation of the inching switch to open position inserts theshunted portion of said second adjustable resistor in the circuit oisaid iirst reactor-saturating winding, a third adjustable resistor inthe circuit oi said first reactor-saturating winding, a normally closedcontactor shunting at least a portion of one of said resistor sections,normally open contactors for shunting the speed control resistor in thesecondary circuit, electromagnetic means for effecting the opening ofthe rst named contactor to recalibrate the effect of said iirstreactor-saturating winding, said electromagnetic means effecting closingof said second named contactors for shunting the resistor sctions in thesecondary circuit, a second of said reactor-saturating windings forproviding a control component to the saturable reactors proportional tothe load current of the motor, a third of said reactor-saturatingwindings for producing a control component that is a function of theprimary terminal Voltage and a function of the alternating currentvoltage supplied to said main winding, said speed control resistors inthe secondary circuit and the capacitors included in the secondarycircuit being operable to change the shape of the speed-torquecharacteristic of the motor.

'7. In a control for a three-phase wound rotor induction motor having athree-phase primary winding and a three-phase secondary winding, thecombination of, circuit connections for applying a three-phasealternating current voltage -to said primary winding, control meansconnected with said circuit connections for control- 3 ling the supplyof electrical energy to said motor, variable impedance means connectedwith said control means for controlling said control means, impedancemeans connected in said three-phase secondary Winding, said impedancemeans comprising a resistive component and a capacitive component,electro-magnetically operated switching means for shunting at least aportion of said resistive component upon operation thereof, switch meansmechanically controlled by adjustment of said variable impedance meansfor operating said electro-magnetically operated switching means,recalibrating impedance means connected in series With said Variableimpedance means, and control means on said electro-magnetically operatedswitching means normally shunting at least a portion of saidrecalibrating impedance means.

8. In a control for a three-phase wound rotor induction motor having athree-phase primary winding and a three-phase secondary winding, thecombination of, circuit connections for applying a three-phasealternating-current Voltage to said primary winding, control meansconnected with said circuit connections for controlling the supply ofalternating current voltage to said primary winding, resistance meansconnected with said three-phase secondary winding, adjustable impedancemeans connected with said control means for controlling the impedance ofthe latter, switching means normally7 shunting at least a part of saidadjustable impedance means, said switching means being operable to shuntat least part of said resistance means and to open the shunt around saidpart of said adjustable impedance means, and means operated byadjustaccosta` ment of said adjustable impedance means for operatingsaid switching means.

9. In a control for a three-phase wound rotor induction motor having athree-phase primary winding and a three-phase secondary winding,secondary circuit impedance means connected in circuit with saidsecondary winding, variable impedance means connected in circuit withsaid primary winding for controlling a vsupply of alternating currentthereto, adjustable control means for controlling the impedance of saidvariable impedance means, and switching means responsive to adjustmentof said control means and having contact means for simultaneouslydecreasing the impedance of said secondary circuit impedance `means andrecalibrating said adjustable control means.

10. In a control for inching operation of an induction motor, thecombination of, reactor 0 means adapted for connection 1n series withsaid motor, control winding means for said reactor means, impedancecontrol means connected in series with said control winding means tocontrol energzation of said control winding means, an inching switchnormally shunting at least a portion of said impedance control means, amain switch for establishing an energizing connection for said reactormeans and said motor, and electrical means controlled by said inc'hingswitch for closing said main switch upon operation of said inchingswitch.

11. In a control for an induction motor having a primary circuit and asecondary circuit, the combination of, impedance means in said secondarycircuit, reactor means connected in said primary circuit forcontrolling' the excitation thereof, control lwinding means for saidreactor means, a rheostat for controlling the excitation of said controlwinding means, resistance means connect-ed in series with said rheostat,a relay having normally open contacts operable when the relay isenergized to shunt at least a portion of said impedance means in saidsecondary circuit, a switch controlled by said rheostat to energize saidrelay upon movement ci said rheostat in a direction to increase thespeed of said motor,

and slow speed switch means having contact means normally shunting atleast a part of said thereof, deenergizing said relay, shunting said 12rheostat and inserting said resistance in the circuit of said controlwinding means.

12. In a control for an induction motor, means for controlling th-eterminal voltage oi said motor comprising, reactance means, reactancecontrol winding means for controlling the reactance of the reactan-cemeans, a transformer having a primary winding connected to the inputsideof said reactance means and having a secondary winding, a reactorhaving a main winding and a pair of control windings, full-waverectifier means, circuit means connecting said secondary winding inseries with said main winding across the input of said full-waverectifying means, said reactance control winding means being connectedacross the output of said ull-wave rectifying means, rectifier meansconnecting one of said pair of control windings to the input of saidreactor means, and rectifier means connecting the other'of said pair ofcontrol windings to the output of said reactor means.

13. In a controlfor an induction motor, means for controlling theterminal voltage of said motor comprising, reactance means, reactancecontrol winding means for controlling the reactance of the reactancemeans, a, transformer having a primary winding connected to the inputside or" said reactance means and having a secondary winding, a reactor`having a main winding and a pair of control windings, full-wave rectiermeans, circuit means connecting said secondary winding in series withsaid main winding across the input of said full-wave rectifying means,said reactance control winding means being connected across the outputof said full-wave rectiiying means, rectiiier means connecting one ofsaid pair of control `windings to the input of said reactor means,rectifier means connecting the other of said pair of control windings tothe out put of said reactor means, and current responsive means havingan input connected to the input oi said reactance means and an outputcon nected to said react/ance control winding means.

References Cited in the rile of this patent UNITED STATES PATENTS NumberName. Date 2,386,580 Wickerham Oct. S, 1945 2,433,153 Pell et al Dec.23, 19417 2,519,196 Pell Aug. 15, 1950

